A magnetic sensor using a Hall element is not only used in a proximity sensor, a linear position sensor, a rotation angle sensor, and the like as a sensor for detecting position information of a magnet, but also widely used in a current sensor for detecting a magnetic field induced by a current flowing through a current conductor to thereby contactlessly measure the current flowing through the current conductor.
Particularly in a current sensor used to detect an inverter current of a motor, a current of an inverter switched at a high frequency needs to be detected with high accuracy, for the purpose of achieving efficient motor control.
The Hall element has a magneto-electric conversion function of generating a Hall electromotive force signal corresponding to an intensity of an input magnetic field, and so is widely used as a magnetic sensor. However, the Hall element has an offset voltage (unbalanced voltage) that is a finite nonzero voltage output even in a state where the magnetic field is zero, i.e. in a magnetic field-free state.
Accordingly, for the magnetic sensor using the Hall element, a Hall element driving method commonly known as a spinning current method or a connection commutation method is available with the aim of canceling out the offset voltage of the Hall element. In this method, an operation of periodically switching, according to a clock called a chopper clock, between a position of a terminal pair for flowing the drive current through the Hall element and a position of a terminal pair for detecting the Hall electromotive force signal is performed (for example, see Non-Patent Document 1 and Patent Document 2).
This spinning current method aimed at cancelling out the offset voltage can be implemented using a switching circuit in a CMOS semiconductor circuit. Hence, a Hall electromotive force signal detection circuit for realizing a high-accuracy magnetic sensor typically includes a switching circuit for implementing the spinning current method.
In a case where the magnetic sensor using the Hall element is applied to the measurement of the inverter current, the magnetic sensor is required to have properties such as a broadband property regarding signal bandwidth, a fast response property regarding signal processing delay time, and a low noise property regarding signal quality. In such a case, as a circuit system for processing the Hall electromotive force signal generated in the Hall element, a continuous-time signal processing circuit that performs signal processing in continuous time is advantageous over a discrete-time signal processing circuit that performs discrete-time processing (sampling). The continuous-time signal processing circuit is free from a phenomenon of folding noise caused by discrete-time processing (sampling), and therefore is a circuit structure especially suitable for use in an environment where significant high-frequency noise is caused by switching the inverter.
The spinning current method of the Hall element will be described below, with reference to FIG. 1A and FIG. 1B.
FIG. 1A and FIG. 1B are diagrams illustrating Hall electromotive force detection when the direction of the drive current for biasing the Hall element is switched between 0 degree and 90 degrees each time the phase of the chopper clock is switched between two values of φ1 and φ2 respectively. The Hall element in the drawing is modeled as a four-terminal element composed of four resistors (R1, R2, R3, and R4), and is driven by a constant current.
It should be noted that, in the model of the Hall element illustrated in FIG. 1A and FIG. 1B, the four resistors (R1, R2, R3, and R4) do not have fixed resistances. In a case where Hall elements are formed as N-wells in a semiconductor substrate, an impurity concentration distribution is developed in each Hall element typically due to a process gradient during semiconductor manufacture. Accordingly, an electric potential distribution in the Hall element (N-well) varies and also a depletion layer developed in the Hall element (N-well) varies depending on from which of the four terminals (terminals 1, 2, 3, and 4) the drive current is applied. Therefore, the resistances of the four resistors R1, R2, R3, and R4 in the model of the Hall element vary depending on from which of the four terminals (terminals 1, 2, 3, and 4) of the Hall element the drive current is applied.
In FIG. 1A and FIG. 1B, voltage signals Vhall(φ1) and Vhall(φ2) measured respectively when the phase of the chopper clock is φ1 (the drive direction of the Hall element is 0 degree) and when the phase of the chopper clock is φ2 (the drive direction of the Hall element is 90 degrees) are each represented as a sum of a Hall electromotive force signal Vsig(B) corresponding to a magnetic field B to be detected by the magnetic sensor using the Hall element and an offset voltage Vos(Hall) of the Hall element, as shown by Expression 1.
Here, the Hall electromotive force signal Vsig(B) corresponding to the magnetic field to be detected can be inverted in polarity by periodically switching the direction of the bias current of the Hall element between 0 degree and 90 degrees according to the phase of the chopper clock, so that Vsig(B) can be frequency-modulated to a frequency f chop of the chopper clock. On the other hand, the DC offset voltage Vos(Hall) of the Hall element remains in the same polarity even when the drive direction of the Hall element is switched between 0 degree and 90 degrees, and thus Vos(Hall) is not frequency-modulated by the chopper clock.
                              Expression          ⁢                                          ⁢          1          ⁢                      :                          ⁢                                                                                                                ⁢                              Signal            ⁢                                                  ⁢            generated            ⁢                                                  ⁢            in            ⁢                                                  ⁢            Hall            ⁢                                                  ⁢            element                    ⁢                                          ⁢                      {                                                                                                      Vhall                      ⁢                                                                                          ⁢                                              (                                                  ϕ                          ⁢                                                                                                          ⁢                          1                                                )                                                              =                                                                  +                                                  Vsig                          ⁡                                                      (                            B                            )                                                                                              +                                              Vos                        ⁡                                                  (                          Hall                          )                                                                                                                                                          (                                                                  when                        ⁢                                                                                                  ⁢                        chopper                        ⁢                                                                                                  ⁢                        clock                                            =                                              ϕ                        ⁢                                                                                                  ⁢                        1                                                              )                                                                                                                                          Vhall                      ⁢                                                                                          ⁢                                              (                                                  ϕ                          ⁢                                                                                                          ⁢                          2                                                )                                                              =                                                                  +                                                  Vsig                          ⁡                                                      (                            B                            )                                                                                              +                                              Vos                        ⁡                                                  (                          Hall                          )                                                                                                                                                          (                                                                  when                        ⁢                                                                                                  ⁢                        chopper                        ⁢                                                                                                  ⁢                        clock                                            =                                              ϕ                        ⁢                                                                                                  ⁢                        2                                                              )                                                                                                          [                  Math          .                                          ⁢          1                ]            
As a result, in the case of performing the operation of switching the direction of the drive current of the Hall element between 0 degree and 90 degrees according to the phase of the chopper clock, the signal Vhall generated in the Hall element has a waveform as illustrated in FIG. 2A to FIG. 2D. Besides, the signal generated in the Hall element has a spectrum as illustrated in FIG. 3, from which it can be understood that the Hall electromotive force signal Vsig(B) corresponding to the magnetic field to be detected and the offset voltage Vos(Hall) of the Hall element are separated in the frequency domain. This is the principle of offset cancellation of the Hall element by the spinning current method.
Moreover, for example, Patent Document 1 discloses that the direction of the drive current is switched clockwise in one of two Hall elements and switched counterclockwise in the other Hall element, with regard to the order and sequence in the spinning current method.
Patent Document 3 relates to a chopped Hall sensor that alternately switches flow of a Hall element excitation current from one direction to another direction, and discloses especially a Hall sensor where a chopped sample-and-hold Hall voltage circuit is clocked synchronously with the switched Hall sensor.
Non-Patent Document 2 discloses a circuit system that performs discrete-time processing (sampling) such as sample and hold, as a circuit structure for realizing the spinning current method of the Hall element.